CURVATURE MEASUREMENTS OF STRESSED SURFACE-ACOUSTIC-WAVE FILTERS USING BRAGG ANGLE CONTOUR MAPPING

摘要

Surface-acoustic-wave (SAW) transversal filters consist of piezoelectric single-crystal quartz plates (dies) with interdigital thin film transducers. The performance of these devices is critically dependent on the spacing and geometry of the interdigitation. Any manufacturing process errors that affect the geometry of the transducers and/or the surface acoustic wave speed will affect the device filter frequency. The presence of stresses or other crystalline defects in the dies can effect the SAW velocity and consequently the frequency of the SAW filter. Bragg angle contour mapping, performed with a double-crystal diffractometer, was used to determine the radii of curvature of single-crystal SAW substrates and test specimens. The angular increment and spacing between contours was then used to calculate the radius of curvature ? from which the stress could be determined. Grit blasting of the back side of the substrates greatly increased the curvature of the dies (R < 5 m) thereby producing stresses, which could be a source of long term frequency drift as the stresses gradually relax. The adhesive used to mount the dies to a pedestal in the device package also produced deformations in the dies. Both of these effects were more pronounced with thinner substrates. Based on these observations, the grit blasting procedure was eliminated and thicker substrates were used to fabricate the SAW devices. As a result, the radii of curvature of devices manufactured under these conditions were larger (R >100 m) than could accurately be measured by the Bragg angle contour technique and the stress levels were greatly reduced.